This invention relates to ultrasound diagnostic scanning where ultrasonic energy illuminates internal organs of the human body in real time and echoes received from the soft organ tissues or from moving scatterers are transduced into electrical signals and then processed to form two-dimensional cross-sectional images that are displayed upon a TV monitor or like display device.
Ultrasound medical systems known as phased arrays have been used for some time and have been described, for example, in U.S. Pat. Nos. 4,140,022 and 4,550,607. Two basic scan and display formats have generally been used in combination with planar linear transducer arrays, that is arrays in which the face of individual transducer elements are positioned in a plane parallel to each other and generally have uniform element spacing.
Two-dimensional images have been formed by linear scanning where ultrasonic beams on parallel acoustic lines normal to or at an angle to the face of a transducer array are propagated by single transducer elements or by selected groups of transducer elements shifted across the array. Linear scanning with parallel lines has the field of view determined by the physical aperture of the transducer array 1 such as width W in FIG. 1. For such a format, the width of the field of view 5 (FOV) is equal to the transducer width W for all scan depths D. Thus, the field of view in the linear scanning format is defined completely by the physical characteristics of the array and is limited by the physical edge of the array. A large field of view requires a large physical aperture of active transducer elements which may create problems of access and good skin contact.
The other scan and display format which is typically used for planar linear transducer arrays is a sector. In a sector format, the elements are spaced much closer together, typically at half-wavelength or so intervals. This permits the acoustic scan lines to be steered without generating grating lobes and allows both the size of the transducer array to be decreased and the field of view to be increased. For example, as shown in FIG. 2, for a 90.degree. sector, the field of view 2 at a scan depth D is given by FOV=2D. Typical scan depths range from 6 cm to 24 cm and are user selectable. Sector phased arrays form acoustic scan lines effectively all originating from the center of the face of the transducer array. The allowable scan angle is a function of the spacing of the individual transducer elements relative to operating frequency. As a consequence, the field of view is also largely defined by the physical characteristics of the array and the field of view vanishes to zero at the face of the array, itself.
A "trapezoidal" scanning and display format has been described in U.S. Pat. No. 4,664,122, which is specific to a particular planar linear array construction. It consists of three sub-arrays, including a central sub-array with substantially larger element spacing, and two end sub-arrays with substantially smaller element spacing. There is a fixed relationship between the element spacing of the central sub-array compared to the end sub-array such as a factor of 2. Acoustic scan lines emanating from the central sub-array and from portions of the two end sub-arrays are parallel to each other, are perpendicular to the transducer face, and are consistent with a linear scan format. Acoustic scan lines emanating from a single point on the face of each end sub-array comprise a left-half and right-half sector. The described end sub-arrays have smaller element spacing, approximately one-half of the transducer wavelength, in order to permit steering the acoustic beams out to angles of approximately 45.degree. with acceptable performance.
Since most transducers which are designed for medical ultrasound imaging systems have uniform element spacing, this "trapezoidal" scanning technique is not advantageous because it does require special transducers with non-uniform element spacing. Transducers which are specifically designed for this format are typically larger than those designed for "sector" scanning. The trapezoidal scan format is described as a compromise between linear and sector scanning, having the advantage of steering, but with the disadvantage of increased transducer size. This size disadvantage is inherent because the increase in the field of view of the displayed image compared to a sector image is increased only to the extent that the transducer is increased in size over that for a corresponding sector transducer. The scan format also is specifically linked to the transducer array construction in contrast to the invention disclosed herein where the scan format is alterable under software control in order to optimize it for different imaging applications using the same transducer geometry.
The physical characteristics of curvilinear arrays also define a field of view which is limited by acoustic lines normal to the face of the array at the end transducer elements. The field of view can be increased by a smaller radius of curvature. However, resolution is impaired and thereby degraded as compared to a less curved array. Also, a large field of view in the near-field requires a large aperture of active transducer elements. A large field of view in the far-field requires some combination of a large aperture and/or a smaller radius of curvature with the attendant loss of resolution.
All the foregoing formats, as well as mechanical and waterpath scanning, have the field of view defined completely by the physical characteristics of the array. In none of these prior art scanning formats is the field of view expanded by situation-dependent software control.